Closed loop feedback systems are well known in the art as a means of controlling process variables in manufacturing facilities, for example the positioning of an actuator to a desired position. Feedback mechanisms and control algorithms are used to drive a perceived error signal to a minimum value. However the premise of feedback control is that the feedback position is that of an error free state.
When sensors are used to monitor actuators, normally the sensors are single element feedback devices. Such sensors are able to provide good feedback either under dynamic conditions or near static conditions, but usually not both. The level of precision and accuracy required from such sensors, particularly those capable of broadband operation, often exceeds what is commercially available at costs that are practical for use in industrial control applications.
When actuator sensors fail abruptly, the failures are generally easily detected with the unfortunate result that feedback from the failed sensors is totally unusable. The only course of action in this instance is to prevent any further control actions of the actuator in order to prevent potentially catastrophic or permanent damage to system components, which severely disrupts the performance of the machine on which the actuators are being used.
When sensor noise levels are near the signal level required for control, closed loop feedback systems may induce oscillations in the controlled process. In paper making machines, particularly sensitive headbox control systems have been observed to experience this phenomenon. Currently the solution to oscillation problems is to widen control deadbands, which results in reduction of precision in the control and slower process response times.
It has also been observed that very slowly occurring drifts or errors, such as those displayed by progressively deteriorating sensor hardware, can be hidden within the closed loop control typical of actuators using feedback control. Such hidden drifts or errors can render the control mechanism and/or typical alarming structure totally useless.